Radiation survey method



Aug. 2, 1960 J. w. MERRITT RADIATION SURVEY METHOD Filed April 12, 1955 uvmvrox do/m W Mew/v77 UM 6 ATTOR NE 4 0 Przomicms WELLS DRY HOLES 2,947,870 Patented Aug; 2.," 1 960 RADIATION SURVEY METHOD 7 John w. Merritt, 208-209 Midco Bldg., Tulsa, Okla. Filed Apr. '12, 1955, Ser. No. 500,889 a 4 Claims. c1; 250-83) This invention relates to surface surveying methods used in locating and identifying subsurface deposits of petroleum and refers more particularly to an improved method for direct detection of such deposits.

Heretofore, various methods for locating petroleum deposits or structural features favorable to the accumulation of such deposits have been proposed and adopted. The known methods can be conveniently dividedin'to two broad categories:' The first and more prevalent of the two are the indirect methods wherein the primary purpose is to locate subsurface geological structures suitable for containing the petroleum deposit, for example, stratigraphic traps, anticlines, faults and domes. Falling in the second category are the direct methods, which make use of observations and measurements of phenomena which are directly associated with and the direct result of the actual existence of a petroleum deposit without reference to the subsurface geological structure. 'Examples of direct surface surveying methods are the socalled geochemical methods involving soil analysis and the more recent gamma ray methods which'are based upon the existence of variation of radiation intensity in the vicinity of petroleum or natural gas deposits.

The direct methods, if they can be made accurate, Jundoubtedly provide a much more desirable tool than the indirect. The inherent and unavoidable disadvantage in the latter is that even though the results of a survey, for example, a seismic survey, may indicate the presence of structure favorable to the accumulation of petroleum, still there exists no means other than drilling to determine its actual presence. Moreover, the indirect methods decrease sharply in value in locating anticlin-al structures of low relief, structural closures against faults, and stratigraphic traps whether they be depositional sand lenses, unconformities or lenticular zones of porosity resulting from chemical action? The importance of the shortcomings of the indirect methods is clearly revealed when it is realized that some of the largest and most prolific oil pools, such as the East Texas Field, occur in stratigraphic traps. Moreover, with the decline in new discoveries of major fields, particularly in the United States, more and more attention has been devoted to re-examining those areas in which for one reason or another the indirect methods did not indicate any positive results.

One of the first attempts to apply a new technique to 'oil exploration resulted in the development of Geo-Chemistry. Chemical analysis of the soil at spaced points 'in and around known producing fields resulted in a pattern showing a greater hydrocarbon content there as compared with the barren ground surrounding the production. It was also discovered that more frequently .the density'pattern took the form of a ring or halo of high values lyinglapproximately above the margin of the production; and thus, defining the borders of the field. The higher concentration of hydrocarbons in the ring or halo was attributed to gas movement upwardly through the earth from the 2 revised to develop a pattern of water-soluble inorganic minerals which'proved far more'simple and was unaffected by changes in temperature and atmosphere pressure. However several difiiculties still remained which often prevented the obtaining of a recognizable pattern,

the chief among these being the non-uniformity of the distribution of the hydrocarbons or water soluble minerals in the soil. Added to this difiiculty was another important one, namely, the fact that without any previous indication above ground whether oil or gas might be foundor, if so, where the margins might be, the geochemist had no idea whether his sampling would be either excessive or insufiicient in area. As a result, either too much field work is doneor another trip to the field becomes necessary in order to round out the. survey and make the geochemical pattern all inclusive. A new and more'simple'method was thus required.

As hereinbefore noted, it has been recognized that water-soluble mineralconcentration and 'liquid hydrocarbon concentration patterns at .the surface closely approximate one another. It also has been discovered that the presence of the mineral concentrations results in patterns of relatively high gamma ray radiation against the background of the surrounding surface. This latter discovery resulted in the technique of the gamma ray survey in which readings of gamma ray activity close to the surface are taken at points along parallel traverse lines across the area being investigated. The readings of the individual stations are then plotted either as profiles along a single traverse or on area maps to provide a gamma activity contour. By noting the anomalies in the pattern (the gamma ray highs), it theoretically should be possible to discover the ring or' halo efiect discovered in the geochemical surveys and in some cases this theory has been successively borne out.

However, more often than not, the gamma ray readings obtained infield surveying show unpredictable variations in closely adjacent areas which still may liewithin the band of mineral concentration. Also it has turned out that the degree of these variations frequently reaches into the class of the anamolies which serve .to define the location of the halo or rings surrounding the production. As a result the pattern obtained is clouded to a considerable extent, often to the point that it is extremely diflicult, if not impossible, to accurately locate the borders of the field.

A primary'object to the present invention is to provide an improved radiation survey technique in which the radiation pattern obtained accurately reflects the anomalies created by the high concentration of radio active material in the hydrocarbon halo and which suppresses the effect of the variation in radiation activiy caused by changes in soil character from point to point. Another equally important object of the invention is to provide means for determining the relationship between changes in soil character at each gamma ray read- .ing station and the change in the gamma ray values ob.- tained, and to provide a method for applying this relationship to the observed gamma ray activity to thus produce a pattern of relative gamma ray activity which reflects more accurately the variations due to presence or absence of gas movements in the soil. f Still another object of the invention is to providean improved gamma ray survey technique in which the abnormal high and low readings in gamma ray activity caused by changes in the retentivity characteristics :of the soil are corrected and in which the final plot of'the readings reflects the variation due primarily to presencejor absence of concentrations of radioactive mineral substances in the hydrocarbon band or halo. T 7,

Another object of the invention is to provide a survey technique which is simple in operation, can be carried out quickly and yet which'is extremely accurate in result.

Other and further objects together with the features of novelty appurtenant thereto will appear in the course of the following description.

In the accompanying drawings, which are included to illustrate a manner of practicing the invention and the advantages resulting therefrom,

Fig. 1 represents a map of an oil producing. field showing the locations of producing. wells and dry holes on which have been plotted the gamma ray contour lines resulting from a conventional gamma ray survey. The contour lines connect points of equal gamma ray activity and are hereinafter referred to as isogams; and

Fig. 2 represents a map of the area illustrated in Fig. 1, but shows the isogam pattern obtained by employing the method described and claimed herein.

Referring initially to Fig. 1, this map represents a typical example of the pattern obtained by known gamma ray survey techniques. The surface gamma ray readings at spaced points along parallel traverse lines are shown by the numbers below the small. circles, each circle representingthe exact location of the station where the reading was taken. The stations are identified in numerical order, the station identifying number appearing above the circle.

Equipment for obtaining readings of ground gamma ray activity is well known and in general use. Perhaps the most accurate type and the type I prefer is the portable ionization chamber which fundamentally consists of two electrodes immersed in an inert gas contained under pressure in a steel chamber. The electrodes are connected externally to a circuit containing batteries. When gamma rays (which have sufiicient penetrability to pass through the steel walls) enter the chamber and ionize the gas, a current flows in the circuit which is directly proportional to the ionization or intensity of radiation. Ordinarily the current is stepped up through amplified circuits and passed through a meter where the values can be read directly or may be automatically recorded on a chart. It will be understood, of course, that other equipment for measuring gamma ray radiation emanating from the ground may be used and my invention is not limited to the use of any particular type of equipment.

In obtaining a map as exemplified by Fig. 1, the portable gamma ray measuring unit is moved from point to point along the traverse lines, a reading being taken at each succeeding station. As a satisfactory spacing for the stations, I prefer proceeding along parallel traverses 660 feet apart, readings being taken at 330 foot intervals along the traverses. The sides of the field can be closed by taking intermediate 330 foot readings between the traverses at each end, as indicated.

The readings from which the Fig. 1 map was prepared are found in column A of Table I.

Table I Gamma Soil 98-Soil Correction True Valu Station A B D E 98-B 1.228XO A+D 1 285 76 22 26 311 2 287 77 21 25 312 3 272 63 35 42 314 4 275 55 53 52 327 5 278 59 39 47 325 6 289 64 34 41 330 7 285 72 26 31 316 8 277 61 37 44 321 9 275 63 45 64 329 10 276 53 35 42 318 11 2B6 74 24 29 315 12 295 80 18 22 317 13 298 92 6 7 305 14 300 98 O O 300 15 300 98 0 0 300 16 296 98 0 0 296 17 298 98 0 0 298 18 295 98 0 0 295 19 282 98 0 0 282 20 298 94 4 5 303 21 318 98 0 0 T (1512 L-C'ontinued Gamma A Correction Gamma Soil 98-Soi1 Correction True Value Station A B Y O D E 93-13 1.228XO A+D Once plotted on the map, isogam lines are drawn to define the readable pattern of activity. It will be observed from Fig. 1 that the areas in which readings below 280 gamma units are found are represented by the portions of the map free from cross hatching. The areas between isogam lines representing an increase offi-om 280 to 290 are distinguished by diagonal cross hatching. Between isogams of 290 and 300 the area is distinguished by horizontal cross hatching and above 300 by vertical cross hatching.

As is believed at once evident, the pattern obtained in Fig. -1 shows no correlation with the actual production characteristics of the region mapped. As has been hereinbefore noted, the pattern indicating possible producing zones of oil is a band or series of high values surrounding the location of the oil deposit and appearing substantially vertically above the outer limits of the deposit.

Fig. 1, however, shows no conclusive or readable pattern of this character. No halo or parallel band effect is present, and such a pattern would not justify a recommendation of the area as a favorable prospect. However, as shown by the symbols on the map, there is definitely alarge producing field running through the central portion of the area.

I have discovered that the primary reason for the failure of the known gamma ray techniques to adequately reflect the anomalies which indicate the presence or absence of oil deposits is the rather extreme variation in localized areas in soil characteristics with particular reference to the ability of the soil to reain the water-soluble mineral concentrations resulting from gas movements. It is generally agreed that surface gamma ray readings ordinarily measure the radiation only from the radioactive minerals located to a depth of from ten to twelve inches below the surface. The intensity of radiation from concentrations below that depth is suppressed by the shielding effect of the surface layer. The ability of the surface layer .to retain the mineral concentrations resulting from subsurface hydrocarbon activity thus has a direct effect on the gamma ray activity at any point. In loose sandy soils it often happens that even though the mineral concentrations were once present in the sur-' face layer, the continual leaching causing by rainfall,

' flooding and the like carries some or all of the minerals downwardly into lower stratas to a point well below the limits at which their radiation can be measured by surface equipment. On the other hand, in tight cl-ay loams surface moisture has little or no effect and the anomalies in gamma ray measurements accurately reflect the degree to which radioactive minerals have been concentrated. In almost every field the character of the soil varies from point to point between these extremes, and this variation has a marked effect on the results of the gamma ray surveys. 1

' Themap appearing as Fig. 2 is a plo't'of the same area shown in Fig. 1 but prepared in accordance with the method embodying my invention. That this map is in complete accord with the location of the production area is believed evident. The fifteen producing wells are surrounded by a closed ring or halo of high values which clearly delineate the area in which production can be expected. It-will be noted that the abandoned wells lie outside the halo and further drilling in this area would 1 appear to be pointless. p 10 To obtain the Fig. 2'map, the following procedure is carried out. Gamma ray readings are taken at each stationas explained in connection with Fig. 1. How ever, in addition to the gamma ray reading there is taken at each station according to my method a sample of the soil. Preferably it is taken at a six-inch depth with any one of the several conventional-devices employed for such. purposes, for example, a sampling pick or soil auger. The samples are placed in individu-albags or other suitable containers, each bag being marked with the identifying data for the particular station. The samples are accumulated as the survey proceeds and are preserved for later use, as hereinafter described.

Following the completion of the field survey which includes the obtaining of the gamma readings and the soil samples at each station, the next step is the obtaining of a measurement of retentivity characteristics of the soil at the station. For this, the samples collected during the surveyare employed. a

A number of procedures are measuring the retentivity characteristics of the samples may be used. In one preferred method the retentivity is obtained by measuring the conductivity ofthe sample after it has been exposed to and allowed to absorb moisture. Highly porous soils such as loose sands lose by leaching action whatever retain more moisture and will consequently have a lower resistance (or higher conductivity).

In another method, a Weighed quantity of soil is placed in the bottom of a centrifuge tube and is covered with water. Following centrifuging the unabsorbed water is poured off and the soil is again weighed. The difference in weight of course provides a value which can be compared with values obtained from other samples to determine their relative water absorbing and retaining properties. In a third method, a measured volume of water is placed in the centrifuge with a weighed quantity of soil, and the discharge water is caught in the process of spin ning the sample. The figure obtained by subtracting the measured volume of dischargedmoisture from the original volume of water placed with the centrifuge soil sample provides a direct measure of the retentivity of the soil.

In obtaining the Fig. 2 map, the electrical conductivity method was relied upon. Each soil sample is first dried at below 212 F., preferably at 180 F. to remove all moisture and then is ground sufficiently fine to break up clods and lumps but Without destroying itsnatural texture. The sample is then thoroughly dried again and placed in a humidifier maintained at a constant temperature and humidity. By way of example, suit-able values for this are F. and a relative humidity of 80%. Each sample is kept in the humidifier for the same period oftime (say, six hours), after which it is remove-d and immediately tested.

Anysuitable testing equipment adapted to be used in measuring electrical conductivity of fine granular materials may be employed. I prefer a test cell comprising a simple tube of conductive material having in its interior a centrally disposed coaxial rod, also formed of material which is a good conductor. The rod and tubeare insulated from one another and the opposite leads from any suitable source of electrical energy of known potential are connected respectively therewith. The sample apavgerq very clearly the changing character of the soil from station to station across the field, particularly in connection with the ability of the soil to retain the mineral concentrations deposited by gas movement- The analysis or testing of the soil provides the basic data necessary for correcting the gamma ray survey to counteract so far as possible the influence of the soil conditions on the radiation measurements.

In a preferred embodiment of my method, the necessary correction of the original radiation measurements is accomplished through the use of a correction factor reflecting the ratio of the range between maximum and minimum gamma ray readings (taken from Table I) to the range of soil retentivity measurements. In the instant example, the ratio is obtained by dividing the difference between the maximum and minimum gamma readings (329 minus 259) by the range of voltage readings (98 minus 41) which gives a factor of 1.228.

The gamma correction to be applied to the original reading is obtained by converting the variations in retentivity of the soil to gamma units through the use of the correction factor. As stated earlier, the ability of a dry soil sample to absorb moisture from the air is a direct indication of its ability to retain the water soluble minerals which may have been deposited therein by gas movement. Soils which adsorb the least moisture are most likely to loose the water-soluble minerals by the leaching action of rain or flood Waters, while those that are fine, tight and relatively highly absorbent exhibit much better retention qualities. In the electrical testing method the most retentive soils are represented by the highest voltage readings in Table I while the least retentive and highly porous soils have the lowest values. Therefore, in order to convert the readings into values which tend to indicate. the degree of correction in the original gamma reading which must be made, the highest value is accepted as the standard for the area and all values are subtracted therefrom; the differences in each case are listed in column C of Table I. To convert these differences to gamma ray corrections, the figures in column C are multiplied in each case by the correction factor. The resultant (column D) is then added to the original reading (column A) and the sum of the two is the corrected radiation value.

The final step in my method consists of the plotting of the corrected readings (column E) on the area map and the interpolation on the map of the isogam lines in order to delineate the pattern of gamma ray activity. In the specific example herein disclosed, the final corrected readings are taken from column E of Table I and were spotted on the map next to the stations to which they correspond. Isogam lines are then drawn to connect the points of equal radiation, intensity with the result observed in Fig. 2. The band of highs encompassing within it the producing wells reflects the outline of the subsurface deposit, and it Will be seen that this band correlates very closely with the results indicated by the producing Wells and abandoned dry holes.

Essentially the same procedure is carried out when the retentivity characteristics are determined by the use of the non-electrical procedures hereinbefore set forth. In the first of these, that is, when weighed samples of dried and ground soil are placed in a centrifuge tube, covered with water, and after centrifuging and after the free water has been poured off, weighed again, the gains in weight represent the retentivity characteristics of the particular samples. The correction factor is obtained by dividing the difference between the greatest and smallest gain into the radiation range, and the correction value is reached by multiplying each gain value by the correction factor. As in the electrical method, the correction is added to the radiation reading to arrive at the figure to be charted on the map for the particular station. In the volumetric method, the measured water thrown off during centrifuging is used as a basis. However, to properly correlate the. measurements Withthe degree of retentivity, the amount thrown off from each sample is subtracted from the volume originally placed in the centrifuge. Highly retentive soils will retain more water and thus the dif-' ference will be greater than in the case of soils with low retentivity. Again, the range of retentivity measurements is divided into the radiation range to correlate the two as a correction factor, and the factor is utilized in the same fashion as in the other methods.

From the foregoing, it will be seen that I have pro-. vided a method of radiation surveying in which the retentivity characteristics of the soil from point to point are correlated with the radiation measurements to produce a pattern of activity reflecting the true character of the field. My invention is thus one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and Which are inherent to the method.

It will be understood that certain features and subcombinations are of utility and may be employed Without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim:

1. A method of exploring for and locating sub-surface hydrocarbon deposits comprising the steps of measuring and recording the surface radiation intensity of the radioactive constituents of the earth at a plurality of stations in a selected area to obtain for each of said stations a radiation value, sampling the soil for its retentivity characteristics at each station, correlating the retentivity characteristics and radiation values and establishing a corrected radiation value for each station, and charting the corrected values to reveal the corrected pattern.

2. A method of exploring for and locating sub-surface hydrocarbon deposits comprising the steps of measuring and recording the surface radiation iintensity of the radioactive constituents of the earth at a plurality of stations in a selected area to obtain for each of such stations a radiation value, obtaining a representative sample of the soil at each of said stations, determining and recording the relative Water-soluble mineral retentivity charac teristics of said samples, applying to the original radiation values a correction based on the variation from station to station in said retentivity characteristics thereby to obtain a corrected radiation value at each station, and charting the corrected values to visually reveal the corrected radiation pattern.

3. A method as in claim 2 wherein said determination of said retentivity characteristics is accomplished by measuring the relative ability of said samples to absorb and retain moisture.

4. A method as in claim 2 wherein said determination of said retentivity characteristics is accomplished by passing an electric current through each of said samples and measuring the relative conductivity thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,165,440 Bays July 11, 1939 2,269,889 Blau Jan. 13, 1942 2,390,931 Fearon Dec. 11, 1945 2,712,986 Huckaboy July 12, 1955 2,725,281 Bond Nov. 29, 1955 

